A known kind of capacitive environmental sensor is shown in FIG. 1. The sensor includes a plurality of electrodes 4 provided on the surface of a semiconductor substrate 2. Oftentimes, the electrodes 4 are provided as a series of interdigitated fingers. A sensor layer 6 covers the electrodes. The sensor layer has a permittivity ε that is sensitive to the presence of a target substance such as moisture or a gas. Measuring the capacitance of the electrodes can determine the presence of the target substance, since the capacitance of the electrodes is governed at least in part by the permittivity of the sensor layer 6. Depending on the sensitivity of the sensor, an assessment may also be made as to the concentration of the target substance. Typically, the capacitance of the electrode 4 is proportional to the amount of the target substance that enters the sensor layer 6.
Sensors of this kind can suffer from drift. Drift can result from a number of different factors, for example:                changes in the permittivity of the substrate 2 (e.g. caused by water uptake), which can also contribute to the capacitance of the electrodes 4;        the presence of substances in the sensor layer other than the target substance (referred to herein a interferents), which can affect the permittivity of the sensor layer and thus affect the capacitance of the electrodes;        phase transitions or chemical reactions associated with the interferents—these are most likely to occur at an interface between electrodes 4 and the sensor layer 6 (“Water at polymer interfaces”, B. D. Vogt, University of Akron, Summer school 2012) and lead to the build-up of an interface layer 8 which can again affect the capacitance of the electrodes 4.        
A strategy that has been developed to cope with sensor drift involves making differential measurements of capacitance. For instance the capacitances of the electrodes may be compared to the capacitance of a neighbouring set of electrodes covered by a sensor layer that is not sensitive to the target substance. However, in most cases, process and IC size limitations prevent the use of different functional layers on the same die, whereas multi-chip implementations may not be viable due to the variability in the drift between the different chips. Furthermore, for certain target substances, it may not be possible to identify an appropriate pair of sensor layers.
An alternative approach, described in EP 1607739 A1, involves making differential measurements in a sensor having a single sensor layer and multiple layers of electrodes which may be switched to have different electrical configurations in which separate measurements of capacitance are made. The change in electrical configuration changes the field line distribution within the sensor. This solution suffers however from the relatively complicated layout of the electrodes (in particular the need to provide electrodes in different layers), which can increase manufacturing costs and constrain design freedom in other parts of the substrate.
U.S. Pat. No. 8,633,047 B2 describes a sensor chip comprising a substrate. A plurality of electrode elements is arranged at a first level on the substrate with at least one gap between neighbouring electrode elements. A metal structure is arranged at a second level on the substrate, wherein the second level is different from the first level. The metal structure at least extends over an area of the second level that is defined by a projection of the at least one gap towards the second level.
EP 1607739 A1 does describe an example having a single level of electrodes. However, as will be explained herein, known solutions having a single level of electrodes are generally ineffective, as they suffer from a lack of sensitivity to the target substance.